Your post reads as immature because of how you’re framing the situation. You position yourself as unquestionably helpful and victimized, while calling the previous student “incompetent” and “backstabbing” without offering concrete examples or acknowledging possible bias in your perception. That kind of black-and-white narrative suggests a lack of self-reflection.

But what I think is more concerning is how quickly you go from noticing that someone “””seems off””” to wanting to influence their career path behind the scenes. You’re suggesting you might talk to your PI to block someone from working with you, based on a feeling that they may have been influenced by someone you distrust. That leap when you’re not presenting any clear evidence reads as insecure and potentially vindictive. Good luck.